Internal frame backpack systems consist of no external framework, rather a variety of semi-flexible inner structures (rod, strut or sheet) are incorporated within the back of the pack with the intention to support or direct the axial load to the lower spine, pelvis, or hips/thighs. However, in actuality a much greater percentage of the axial load is directed to the upper spine, shoulders and neck anatomy as the load typically hangs from the user's shoulders. This is due to the lacking or inefficient system of support by the backpack and the internal framing to direct the axial loads away from the upper anatomy to the more preferable lower spinal, pelvic/hip anatomy. Due to reduced system rigidity, movements and ranges-of-motions of the user may be minimally improved compared to the external frame system, however, the improvement comes at the cost of efficient load support. Given the greatly reduced efficiency and capabilities of load support of the internal frame system, the gear and items that make up the load within the backpack must be packed tightly in an effort to increase interior pressures or tensions to improve support as well as stability. This is only minimally effective and has the negative results of increased wear to the pack materials, and the gear and items that make up the load. The internal frame systems are typically of reduced durability and weight compared to the external frame systems. Internal frame systems typically hang close to the user, impeding airflow, resulting in moisture and heat retention. U.S. Pat. No. 4,479,595 to Opsal discloses an internal backpack with an internal central vertical stay of which is attached at the distal end to a free floating waist band that allows for triplanar motion (frontal/sagittal/transverse). Motion is achieved via the flexible nature of the connector fabric.
U.S. Pat. No. 6,070,776 to Furnary discloses an internal frame backpack in which the length of the shoulder straps automatically adjust to length when the wearer twists his or her torso, thereby allowing limited torso transverse plane (axial rotation) motion. Additionally, the elastic nature of the shoulder straps allows unilateral shoulder/arm motion in the superior direction.
External frame backpack systems consist of an outer frame to which the pack bags are attached. External frame systems are quite rigid. This allows for the ideal of increased support of the load by directing the axial load forces specifically to the lower spinal, pelvic/hip architecture. This results in the support of the axial load by the larger and stronger bony and muscular anatomy of the lower spine, pelvis and hips/thighs, rather than the smaller, weaker upper spinal, shoulder, and neck anatomy. The high rigidity of the frame materials allows greater stability of the load compared to internal frame backpacks. In addition, greater load capacities are possible, due to the high-strength and high-durability of the frame materials when compared to internal frame systems. The rigidity of the external frame system, however, also functions to limit the normal movements and ranges-of-motions of the user during use. The natural dynamics and kinesiology of the user during ambulation are forced to fight the opposing dynamics and strength and rigidity of the external frame. This requires greatly increased energy expenditures by the user, resulting in greatly reduced speed, endurance, comfort and overall satisfaction of the experience. The external frame system is typically of greater weight when compared to the internal frame system. U.S. Pat. No. 5,184,763 to Blaisdell et al discloses a backpack system having upper and lower modules that are connected by a three-axes ball joint assembly. This joint assembly permits free movement of the hips relative to the shoulders in all directions while transmitting the load to the hips.
The external framework and packs are held away from the user, allowing improved airflow, thus improved dryness and heat escape when compared to the internal frame system. Although internal frame backpack systems and external frame backpack systems provide improved loading distribution relative to non-self supporting fabric backpack having no frame, there is still a need for a backpack system that optimally ensures that the load is directed to the user's lower anatomy, which is the portion of the user's anatomy best suited for load support.